How does a capacitor react to AC?
The correct answer is A: As the frequency of the applied AC increases, the reactance decreases. How a capacitor reacts to AC is that as the frequency of the applied AC increases, the reactance decreases. Capacitive reactance XC = 1/(2πfC), so it's inversely proportional to frequency. For amateur radio operators, this is a fundamental relationship. Understanding this helps when working with capacitors.
Exam Tip
Capacitor reactance = decreases with frequency. Think 'C'apacitor 'R'eactance = 'C'ontrary (decreases) with frequency. XC = 1/(2πfC), so reactance is inversely proportional to frequency. Not increases, not amplitude-dependent - just decreases with frequency.
Memory Aid
"Capacitor reactance = decreases with frequency. Think 'C'apacitor 'R'eactance = 'C'ontrary (decreases). XC = 1/(2πfC), so reactance is inversely proportional to frequency. Higher frequency = lower reactance."
Real-World Application
A capacitor with 100 pF capacitance. At 1 MHz, reactance is about 1,590 ohms. At 10 MHz, reactance is about 159 ohms. As frequency increases, capacitive reactance decreases inversely. This is why capacitors pass high frequencies (low reactance).
Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Capacitive reactance doesn't increase with frequency - it decreases. XC = 1/(2πfC), so higher frequency means lower reactance.
Option C: Incorrect. Reactance doesn't depend on amplitude - it depends on frequency and capacitance. Amplitude doesn't affect reactance.
Option D: Incorrect. Reactance doesn't decrease with amplitude - it doesn't depend on amplitude at all. Amplitude doesn't affect reactance.
题目解析
The correct answer is A: As the frequency of the applied AC increases, the reactance decreases. How a capacitor reacts to AC is that as the frequency of the applied AC increases, the reactance decreases. Capacitive reactance XC = 1/(2πfC), so it's inversely proportional to frequency. For amateur radio operators, this is a fundamental relationship. Understanding this helps when working with capacitors.
考试技巧
Capacitor reactance = decreases with frequency. Think 'C'apacitor 'R'eactance = 'C'ontrary (decreases) with frequency. XC = 1/(2πfC), so reactance is inversely proportional to frequency. Not increases, not amplitude-dependent - just decreases with frequency.
记忆口诀
Capacitor reactance = decreases with frequency. Think 'C'apacitor 'R'eactance = 'C'ontrary (decreases). XC = 1/(2πfC), so reactance is inversely proportional to frequency. Higher frequency = lower reactance.
实际应用示例
A capacitor with 100 pF capacitance. At 1 MHz, reactance is about 1,590 ohms. At 10 MHz, reactance is about 159 ohms. As frequency increases, capacitive reactance decreases inversely. This is why capacitors pass high frequencies (low reactance).
错误选项分析
Option B: Incorrect. Capacitive reactance doesn't increase with frequency - it decreases. XC = 1/(2πfC), so higher frequency means lower reactance. Option C: Incorrect. Reactance doesn't depend on amplitude - it depends on frequency and capacitance. Amplitude doesn't affect reactance. Option D: Incorrect. Reactance doesn't decrease with amplitude - it doesn't depend on amplitude at all. Amplitude doesn't affect reactance.
知识点
Capacitor, Reactance, Frequency increase, Capacitive reactance
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.